Previously we always stored 4 bytes of origin at the destination address
even for 8-byte (and longer) stores.
This should fix rare missing, or incorrect, origin stacks in MSan reports.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226658 91177308-0d34-0410-b5e6-96231b3b80d8
